1. Field of the Invention
The present invention relates to a thermal printer, and more particularly, to a thermal printer having a thermal printhead with energy save features.
2. Description of the Related Art
Thermal printing techniques have been widely used in such areas as portable/mobile, retail, gaming/lottery, and medical due to several advantages over other types of printing techniques such as inkjet, laser or ribbon. Some examples of the advantages are quiet operation, light weight due to a simple structure, no need for ink, toner, or ribbon to replace, and the like. With these advantages, thermal printers based on the thermal printing techniques are built into a variety of devices including battery operated devices which may need to operate in an extreme environment. In particular, thermal printers in such devices are likely to be subjected to a wider range of temperatures compared with other types of printers which are mainly used in offices or in a house. As thermal printers rely on heat to print images onto a thermosensitive paper, there is a need for a thermal printhead used in a thermal printer that can offer a reliable fast printing without deterioration of the printing quality even in an extreme ambient temperature. In addition to such a need, there is also a need for a thermal printer that can offer a long battery life for battery operated devices.
FIG. 1 shows a simplified cross-sectional view of a conventional thermal printhead B1. The thermal printhead B1 includes a substrate 101, a resistor layer 102, a heatsink 105, a drive IC 106 and a platen 120. In printing an image using the thermal printhead B1, a portion of the resistor layer 102 which constitutes a heating element to imprint a dot is heated by supplying electrical power. When a series of dots is to be printed, this particular portion of the resistor layer 102 is repeatedly supplied with electrical power with power on times in between power off times and the series of dots is printed onto a thermosensitive paper 121 during the power on times. If the series of dots is a long one, the temperature buildup of the resistive layer 102 may occur. Particularly, when On/Off switching speed of supplying electrical power is increased, it may become difficult for the resistive layer 102 to follow the increased switching speed because the resistor layer 102 cannot dissipate the heat fast enough due to the temperature buildup.
In contrast to forced heating of the particular portion of the resistor layer 102 by electrical power, cooling of the particular portion of the resistor layer 102 occurs by conducting heat through the substrate 101 and by dissipating the heat through the heatsink 105 to surrounding air. In other words, cooling time of the heating element of the resistor layer 102 depends on natural cooling which in turn depends on such factors as the combination of the heat capacity of the resistor layer 102, heat capacity and conductivity of the substrate 102 and the heatsink 105 and an ambient temperature of the surrounding air. If, for example, the heat capacities of the resistor layer 102 and the substrate 101 are too large to dissipate the heat in time to follow the On/Off switching speed, problems such as trailing or a blur of a printing dot may occur. Even if the heat capacities of the resistor layer 102 and the substrate 101 are small, if the heatsink 105 cannot dissipate the heat conducted by the resistor layer 102 and the substrate 101 fast enough, the same problems may occur. This extra heat which needs to be dissipated, not only causes problems in printing, but also the electrical energy used to generate the extra heat is entirely wasted from the perspective of the device power source.